Even experts are surprised at the vast numbers of U.S. soldiers who have been deployed after 9/11. Even if troop levels in Iraq are cut next year, the military may be permanently damaged.

Three and a half years have passed since U.S. bombs started falling in Afghanistan, and ever since then, the U.S. military has been engaged in combat overseas. What most Americans are probably unaware of, however, is just how many American soldiers have been deployed. Well over 1 million U.S. troops have fought in the wars since Sept. 11, 2001, according to Pentagon data released to Salon. As of Jan. 31, 2005, the exact figure was 1,048,884, approximately one-third the number of troops ever stationed in or around Vietnam during 15 years of that conflict.

More surprising is the number of troops who have gone to war since 9/11, come back home, and then were redeployed to the battle zone. Of all the troops ever sent to Iraq or Afghanistan, one-third have gone more than once, according to the Pentagon. In the regular Army, 63 percent of the soldiers have been to war at least one time, and almost 40 percent of those soldiers have gone back. The highest rate of first-time deployments belongs to the Marine Corps Reserve: Almost 90 percent have fought.

The data sheds new light on how all-consuming the post-9/11 wars have been for the U.S. military, and suggests a particular strain on U.S. ground forces. An increasing number of military experts believe those forces -- the Army and Marines -- are months away from being overtaxed to the point of serious dysfunction. The situation in Iraq must continue to stabilize. If it doesn't, and the Bush administration continues to both reject the idea of a draft and rebuff efforts to permanently increase the size of the Army and Marines, U.S. ground forces will break down to a point not seen since just after Vietnam.

"Unless things start to improve, we will start to see a serious problem in six to nine months," said Bernard E. Trainor, a retired Marine Corps three-star general and a former Marine Corps deputy chief of staff under Ronald Reagan. "I think they [the Pentagon] are betting that things are going to get better. But that could be a miscalculation," said Trainor. "This crowd has been pretty good at miscalculating."

Indeed, the revelation that well over 1 million U.S. troops have fought in Iraq and Afghanistan surprises even close military observers. "Those are big numbers ... a lot bigger than I would have thought," said John Pike, the director of GlobalSecurity.org, a defense information Web site that tracks the logistics of war. Pike thinks it is too early to tell what the impact will be on the regular Army, but he said the repeated deployments have already broken the reserve forces.

The particularly grinding service in Iraq puts a special brand of wear and tear on the troops, as evidenced in, among other things, the rate of mental illness among soldiers coming home. Among veterans who served in Iraq and are now seeking healthcare from the Department of Veterans Affairs, one in four is now being diagnosed with a mental problem, according to a recent study in the New England Journal of Medicine. There are no front lines in Iraq: Transportation companies, military police and civil affairs soldiers face the same risk of random ambush or death by roadside bombs. The stress goes on 24 hours a day for an entire tour. (Tours vary by unit, with some Army soldiers serving up to a year per tour, and Marines serving seven-month tours.) Veterans of Vietnam say some of that sounds eerily familiar.

During the 15 years of the Vietnam conflict, around 2.4 million troops served there, according to a study of Pentagon data by the Heritage Center for Data Analysis. Some estimates put another 1 million troops in surrounding countries during that time. The U.S. started moving new troops into the Vietnam arena in 1956 and troop levels peaked in Vietnam in 1968 when nearly a half-million troops were there. Most news reports about current military engagement focus on the number of troops in Iraq now: 150,000 are there, with another 20,000 on the ground in Afghanistan, according to the Pentagon.

The United States drafted nearly 2 million people during the Vietnam War era, according to the Selective Service System, but did not activate military reserves as the military is doing for the Iraq war. But no one in the Bush administration has uttered the "D" word for this war. Under intense pressure from Congress, the Bush administration has agreed to temporarily increase the size of the Army until 2008, but says it does not want to permanently increase ground forces because of cost. But if the government does nothing to alleviate the strain on troops, military analysts worry that Iraq might turn into another Vietnam -- but not in the way most people think of that comparison.

Instead, military experts said the tempo of the Iraq war will eventually erode the Army and the Marine Corps into a state of disrepair similar to that after Vietnam, when discipline, morale and readiness were considered by some historians to be the worst ever. The Army was recovering from a war in which troops had killed their superior officers. Drugs were rampant. Some units in Vietnam had refused to fight. That took a decade to fix as the military moved away from the draft to an all-volunteer force in 1973 and began to purge officers who were performing poorly.

Some factors that contributed to the post-Vietnam military slump were particular to that conflict and do not apply to Iraq; most notably, the Vietnam-era Army included a large number of conscripted soldiers. The modern professional soldier is more motivated and better trained. Conventional wisdom says that the modern all-volunteer Army can last longer in war and bounce back faster. But the risk of pushing the military too far still remains.

Anecdotes and examples abound showing the current strain on the military. The Iraq war is burning through troops. In addition to troops getting treatment in military hospitals, nearly 50,000 veterans from Iraq and Afghanistan, including those discharged for wounds or injuries, are now out of the military and getting medical treatment from the Department of Veterans Affairs, according to V.A. data. Around 25,000 troops have been medically evacuated from Iraq and Afghanistan, according to the Pentagon's transportation command.

Further, calls to the GI Rights Hotline, an 800 number set up by nonprofit groups for soldiers to get information on military discharges, have nearly tripled since the year 2000. The hot line got 32,200 calls last year from soldiers who don't want to go to Iraq -- or don't want to go back. "The majority of the calls are people who are trying to get out," said the hot line's manager, Steve Morse, GI rights program coordinator for the Central Committee for Conscientious Objectors in Oakland, Calif. Most of the calls are from AWOL soldiers who are looking for help, or are interested in becoming conscientious objectors or getting some sort of discharge. A February Harper's article said 5,500 troops have gone AWOL since the invasion of Iraq.

The good news is that the situation in Iraq may be genuinely improving. The Pentagon reported last week that the number of "terrorist incidents" in Iraq has dropped to the lowest level since March 2004. The rates of combat deaths, which have fluctuated since the invasion in March 2003, are decreasing this year. But that could change at any time. And now the American military is at a precarious tipping point. Even some current Pentagon leaders have expressed concern. "What keeps me awake at night," Army Vice Chief of Staff Gen. Richard A. Cody told a Senate panel last month, "is what will this all-volunteer force look like in 2007?"

Pentagon officials told the New York Times recently that the United States might be able to reduce the number of troops in Iraq to 105,000 early next year -- if violence does not spike again. (The article notes that similar plans last year were put on ice after the insurgency heated up.) Some military experts said by early next year, it will already be too late to prevent serious damage to U.S. ground forces.

"If you want to ask how to destroy the all-volunteer Army, the Bush administration has provided a textbook case," Lawrence J. Korb told an audience at a Center for American Progress debate on the draft this month. Korb, an assistant secretary of defense under Reagan, said the strain may soon become overwhelming -- and Bush is not doing enough about it. "It may be that at some point we have cracked the all-volunteer force so much, we will have to do something else." Korb said that he thinks that three combat tours is the breaking point. Some combat units, such as the Army's famed Third Infantry Division, are in Iraq for the second time now.

Ironically, while some experts think the draft exacerbated the desolation of the Army after Vietnam, others argue that it is one option to maintain national security given the current strain on the all-volunteer force. "America has a choice. It can be the world's superpower or it can maintain the current all-volunteer military, but it probably can't do both," Phillip Carter and Paul Glastris wrote in the Washington Monthly last month.

The Pentagon has moved to stop the bleeding, enacting "stop loss" policies that prevent some soldiers from leaving the military. They have tapped the Individual Ready Reserve, soldiers who thought they had severed ties with the military years ago. Critics have said these policies are part of a "back door draft." The Bush administration has agreed only to the temporary increase in the size of the Army until 2008 and is reconfiguring combat units to get more foot-soldier bang for its buck.

But recruitment is also falling, particularly for Army Reserve units. The Pentagon said last month that both the active-duty and reserve forces are behind on recruiting goals for this year. The National Guard is down 25 percent. The Pentagon is adding new recruiters to try to fill the gaps: The Army National Guard has said it will add another 1,400 alone. "This will be a very challenging year for recruiting for the reserve components," Charles S. Abell, principal deputy undersecretary of defense for personnel and readiness, told Congress last month. That trend continues even as the military increases signing bonuses and lowers its standards for signing up. (Most recently, the government decided that a new recruit into the reserves could be 39 instead of 34.)

Sen. Chuck Hagel, R-Neb., and Sen. Jack Reed, D-R.I. -- a Vietnam veteran and an Army Ranger, respectively -- want to permanently increase the Army by 30,000 soldiers and add 3,000 Marines. The Bush administration has balked at such efforts, citing the $3 billion price tag. The most pressing issue may be the reserves. Fearing a political backlash if he deployed weekend warriors to Vietnam, President Lyndon Johnson bypassed the reserves and used the draft instead. Indeed, slots in the National Guard were particularly coveted during that era, as the 2004 presidential election and the revisiting of George W. Bush's Guard record made so clear.

After Vietnam, the Pentagon reorganized the military so that it can't fight a big ground war without mobilizing the reserves. The idea was to block the president from waging a war without the full support of the American heartland. Active-duty Army units now rely on reserve units to perform vital functions in a major mobilization.

But the reserves are lagging the farthest behind in meeting their recruitment goals. The long deployments may have been particularly shocking for the troops, many of whom simply did not think they were signing up for this. The grind is wearing the reserves down, and fewer people are willing to sign up for it now. The Army Reserve's chief, Lt. Gen. James Helmly, wrote in a memo to Army Chief of Staff Gen. Peter Schoomaker late last year that the stress meant the Army Reserve was "degenerating into a 'broken' force."

Pike, from GlobalSecurity.org, said the situation for the reserves is dire. "The guard is broken and cannot be fixed," Pike said. "I don't think anybody would voluntarily, of their own volition, join the National Guard. I think they will have to come up with a new mission statement for the thing."